The present invention relates, in general, to circulating fluidized bed (CFB) reactors or combustors and, more particularly, to a CFB reactor or combustor having a selective catalytic reduction (SCR) system employed downstream of the CFB reactor or combustor furnace to achieve enhanced NOx reduction capability.
Environmental protection and the control of solid, liquid and gaseous effluents or emissions are key elements in the design of steam generating systems which utilize the heat generated by the combustion of fossil fuels to generate steam. At present, the most significant of these emissions are sulfur dioxide (SO2), oxides of nitrogen (NOx) and airborne particulate.
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other species generated during combustion. Once the fuel is chosen, NOx emissions are minimized using low NOx combustion technology and postcombustion techniques. If combustion modifications alone are insufficient, postcombustion techniques such as selective noncatalytic reduction (SNCR) or selective catalytic reduction (SCR) systems may be employed. In SNCR or SCR systems, NOx is reduced to nitrogen (N2) and water (H2O) through a series of reactions with a chemical reagent injected into the flue gas. Ammonia and urea are most the commonly used chemical reagents with SNCR systems, while ammonia is most commonly used for SCR systems.
Fluidized bed combustion has distinct advantages for burning solid fuels and recovering energy to produce steam; indeed, the primary driving force for the development of fluidized bed combustors in the United States is reduced SO2 and NOx emissions. Typically, this technology can be used to burn high sulfur coals and achieve low SO2 emission levels without the need for additional back-end sulfur removal equipment. Fluidized bed boilers are designed so that the bed operating temperature is between 1500 and 1600xc2x0 F., resulting in lower NOx emissions. These lower operating temperatures also permit combustion of lower grade fuels (which generally have high slagging and fouling characteristics) without experiencing many of the operational difficulties which normally occur when such fuels are burned.
In CFB reactors or combustors, reacting and non-reacting solids are entrained within a reactor enclosure by an upward gas flow which carries the solids to an exit at an upper portion of the reactor enclosure. There, the solids are typically collected by an impact type primary particle separator, and returned to a bottom portion of the reactor enclosure either directly or through one or more conduits. The impact type primary particle separator at the reactor enclosure exit typically collects from 90% to 97% of the circulating solids. If required by the process, an additional solids collector may be installed downstream of the impact type primary particle separator to collect additional solids for eventual return to the reactor enclosure.
CFB reactors or combustors are known (see, for example, U.S. Pat. No. 5,343,830 to Alexander et al.) wherein the two or more rows of impingement members located within the furnace or reactor enclosure are followed by a second array of staggered impingement members which further separate particles from the gas stream, and return them via cavity means and particle return means without external and internal recycle conduits.
Both SCR and SNCR systems have been applied to reduce NOx , emissions from pulverized coal fired steam generating systems. SNCR systems have also been applied to fluidized bed steam generators, and it has been proposed to combine a CFB steam generator for petroleum coke firing with an SCR system.
The present invention relates generally to the field of circulating fluidized bed (CFB) reactors or combustors and provides a system to achieve low NOx emissions at lowest operating cost. Fluidized bed combustion technologies provide combustion temperatures that are much lower (1550-1600xc2x0 F.) at the point of fuel admission than in pulverized coal combustion systems, where the combustion temperatures may be 2500-3000xc2x0 F. This difference in combustion temperature contributes to a large difference in uncontrolled NOx, emissions from the fluidized bed. Uncontrolled NOx, emissions from pulverized coal technologies typically ranges from 0.3 to 0.7 lbs/106 Btu, but NOx, emissions from fluidized bed technologies is several times less, typically 0.12-0.2 lbs/106 Btu. However, even more stringent emissions regulations are being encountered, typically on the order of 0.10 lbs/106 Btu. This degree of NOx reduction has been accomplished on fluid bed technologies with SNCR systems (spraying ammonia at locations where the gas temperatures are in the range of 1450-1650xc2x0 F.), and on pulverized coal technologies with SCR systems (spraying ammonia at locations where the gas temperatures are in the range of 750xc2x0 F.). However, experience with SCR technology has shown that less ammonia is needed for a given reduction in NOx and the unreacted ammonia leaving the system is less than with SNCR technology (usually, 5 ppm with SCR as compared with 25 ppm with SNCR). Since the initial NOx in fluidized bed systems is lower, the NOx after the SCR system can be much lower with only a minimal use of catalyst and ammonia.
Accordingly, one aspect of the present invention is drawn to a combination of a CFB reactor or combustor and an SCR system. The combination comprises a CFB reactor enclosure for conveying a flow of flue gas/solids therethrough, primary particle separator means for separating solids particles from the flow of flue gas/solids, and means for returning the solids particles collected by the primary particle separator means to the reactor enclosure. At least one of superheater and reheater heat transfer surface is located downstream of the primary particle separator means with respect to the flow of flue gas/solids. Multiclone dust collector means, located downstream of the at least one of superheater and reheater heat transfer surface, are provided for further separating solids particles from the flow of flue gas/solids, together with means for returning the solids particles collected by the multiclone dust collector means to the reactor enclosure. An SCR system is located downstream of the multiclone dust collector means for removing NOx from the flow of flue gas/solids, and dry scrubber means is located downstream of the SCR system. Finally, means are provided for injecting ammonia into the flow of flue gas/solids upstream of the SCR system to cause the chemical reactions which reduce the NOx emissions.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.